O2 sensor signal working?

[ronald1855]

91 525. Burned out my O2 sensor heater wire. But the o2 sensor signal works. Tested it from cold, after a couple of minutes of driving normal O2 sensor signal flapping happens. Left it connected with my car thinking its basically just an unheated o2 sensor.

Somebody told me recently that once the car does not see the o2 sensor signal from startup, it assumes the sensor is dead. And even if the correct signal shows up after a few minutes, it ignores it. Some motronic thing. Is this true? Thanks.

[shogun]

Limited Operating Strategy (LOS)
In the event of a serious fault in one or more of the sensors or their wiring circuits, Motronic will substitute a fixed default value in place of the defective sensor. This procedure is often termed limp home. A serious fault occurs when the signal from the sensor is outside of its normal operating parameters. When operating in LOS the engine may actually run quite well with failure of one or more minor sensors. Since the substituted values are those of a hot engine, cold starting and running during the warm-up period may be less than satisfactory. Also, failure of a major sensor, ie the AFS, will tend to make driving conditions less easy. Once the fault has cleared, Motronic will once more accept the live signal from the sensor. The following LOS measures are taken in the event of a failure

Catalytic Converter and emission control
The Motronic injection system fitted to BMW vehicles equipped with a Catalytic Converter implements a closed loop control system so that exhaust emissions may be reduced. Closed loop systems are equipped with an oxygen sensor which monitors the exhaust gas for oxygen content. A low oxygen level in the exhaust signifies a rich mixture. A high oxygen level in the exhaust signifies a weak mixture. The Oxygen Sensor closed loop voltage is quite low and switches between 100 mVolts (weak) to 1.0 volt (rich).
The signal actually takes the form of a switch and switches from weak to rich at the rate of approximately 1 HZ. A digital voltmeter connected to the signal wire, would display an average voltage of approximately 0.45 volts.

In the event of OS circuit failure, the ECU substitutes a constant voltage of 0.45 volts and this should not be confused with the average voltage of 0.45 which occurs during switching from approximately 1.0 volt to 0.1 volt. When the engine is operating under closed loop control, the OS signal causes the ECU to modify the injector pulse so that the AFR is maintained close to the stoichiometric ratio. By controlling the injection pulse, during most operating conditions, so that the air/ fuel ratio is always in a small window around the Lambda point (ie Lambda = 0.98 to 1.04), almost perfect combustion is achieved. Thus the Catalyst has less work to do and it will last longer with fewer emissions at the tail pipe. The closed loop control is implemented during engine operation at engine normal operating temperature. When the coolant temperatures is below 70° C, or the engine is at full load or is on the overrun the ECU will operate in open loop. When operating in open loop, the ECU allows a richer or leaner AFR than the stoichiometric ratio. This prevents engine hesitation, for example, during acceleration with a wide open throttle. The OS only produces a signal when the exhaust gas, has reached a minimum temperature of approximately 300° C. In order that the OS will reach optimum operating temperature as quickly as possible after the engine has started, the OS contains a heating element. The OS heater is controlled by the ECU through an OS relay or from the fuel pump relay depending on vehicle. The ECU switches off the OS relay under certain conditions of speed and load.

[ronald1855]

Thanks for the info but i find it unclear.

Limited Operating Strategy (LOS) In the event of a serious fault in one or more of the sensors or their wiring circuits, Motronic will substitute a fixed default value in place of the defective sensor. This procedure is often termed limp home. A serious fault occurs when the signal from the sensor is outside of its normal operating parameters.

Does the ecu consider it a serious fault when the heater does not work, and the sensor does not respond for 5 minutes of normal driving, but then responds normally after that ?

Once the fault has cleared, Motronic will once more accept the live signal from the sensor. The following LOS measures are taken in the event of a failure

So does this mean that as long as there's an error code for something, replacing that with a good unit will not bring it online until the old error code is first cleared ? I was under the impression that the car interrogates all sensors afresh on startup and keeps them online if they pass the self test. But that an error code stays stored for 60 startups without the fault happening again and then it is self deleted.

The signal actually takes the form of a switch and switches from weak to rich at the rate of approximately 1 HZ. A digital voltmeter connected to the signal wire, would display an average voltage of approximately 0.45 volts.

Yes, I see this exactly when I wire in my multimeter to the o2 sensor circuit and feed it into the cabin. It reads 0.45volts on startup and after about 5 minutes of driving, starts flicking between near 0 and near 1 volt. [/quote]

I feel my o2 sensor itself is ok. But there is no continuity for the electrical heater circuit. Its a 4 wire sensor. There is a separate relay for the o2 sensor heater. Its not a shared relay with the fuel pump.

In the event of OS circuit failure, the ECU substitutes a constant voltage of 0.45 volts and this should not be confused with the average voltage of 0.45 which occurs during switching from approximately 1.0 volt to 0.1 volt.

Is a failure in the o2 sensor heater circuit, the same as an "OS circuit failure" as listed above ?

The OS only produces a signal when the exhaust gas, has reached a minimum temperature of approximately 300° C. In order that the OS will reach optimum operating temperature as quickly as possible after the engine has started, the OS contains a heating element. The OS heater is controlled by the ECU through an OS relay or from the fuel pump relay depending on vehicle. The ECU switches off the OS relay under certain conditions of speed and load.

So the o2 sensor heater circuit is clearly "looked at" by the ecu. But does it ignore the o2 sensor itself when the heater circuit is dead, but the sensor starts working normally after 5 minutes of engine running?

[ronald1855]

Bump ?

[shogun]

Be patient and do not push after some hours.

[demetk]

So the Motronic on the 91 525 monitors the heating circuit and throws a fault code? I did not know that.

On my 89 with M1.3 the check engine light turned on because the O2 sensor signal was way out of range. Once I replaced the O2 sensor the check engine light went out and the engine went back to normal. I didn't have to reset anything.

[shogun]

that is from the M1.1 and M1.3 and it should be similar on newer ones, here the cpl. details https://www.bimmerforums.com/forum/s...ic-1-1-1-3-BMW

Once I replaced the O2 sensor the check engine light went out and the engine went back to normal. I didn't have to reset anything.

Once the fault has cleared, Motronic will once more accept the live signal from the sensor.
But the fault is still in the system first: The Motronic 1.3 system has a self-test capability that regularly examines the signals from engine sensors and internally logs a code in the event of a fault being present. This code can be extracted from the Motronic Serial Port by a suitable Fault Code Reader. If the fault clears, the code will remain logged until wiped clean with a suitable FCR, or until the engine has been started for more than 5 times - when the fault code is self initialising. An ECU that retains codes for faults of an intermittent nature is a valuable aid to fault diagnosis.
Motronic 1.1 1987 to 1988: The ECU will store a maximum of 5 fault codes.

[ronald1855]

I'm on Motronic 3.1.

[shogun]

I'm on Motronic 3.1.

That does not matter, same principle

[ronald1855]

Thank you. That helps.

Anyway aside from that, I'm pretty sure for the dme 3.1s that fault code is only deleted after like 50-60 good startups and not 5.

Are there two separate faults, an o2 sensor fault, and an o2 sensor heater fault ? If there are, then a sensor heater fault will not stop the car from listening to the sensor itself if it is still undamaged.

Let me google obd1 oxygen sensor codes...

[whiskychaser]

There are 3: 37,70 and 201. Heater relay circuit, sensor circuit and sensor control.
Doubt you will get check engine light with a duff sensor heater circuit